This invention concerns drilling or completion fluids, particularly water-based drilling or completion fluids. More specifically, it pertains to additives for such fluids. Even more specifically, the invention relates to additives used to prevent shales or clayey formations from adversely affecting well drilling or well completion operations.
Drilling fluids are used in well drilling operations, e.g., during drilling of oil and gas wells. During drilling, drilling fluid is pumped down a drillstring, discharged through ports in the drill bit and returned to the surface via the annulus between the drillpipe and the surrounding formation. The drilling fluid performs a variety of functions including cooling and lubricatirg the drill bit and drillstring, removing rock cuttings generated during the drilling process and carrying them to the surface, susperding cuttings in the annulus when pumping stops, preventing squeezing in or caving of the formation and keeping formation fluids at bay.
Drilling fluids generally comprise a carrier, a weighting agent and chemical additives. Drilling fluids fall into two main categories: water-based drilling fluids, also known as water based muds (WBMs), in which the carrier is an aqueous medium; and oil-based drilling fluids, also known as oil-based muds (OBMs), in which the carrier is oil or a water/oil emulsion. Oil based muds are technically superior to WBMs in certain important respects, including the comparative lack of adverse reactivity of OBMs with shales, one of the most commonly encountered rock types during drilling for oil and gas. The use of OBMs, however, has the disadvantage of resulting in production of large quantities of oil-contaminated waste products such as cuttings that are difficult to dispose of in an environmentally acceptable way. While the use of WBMs is environmentally more acceptable than of OBMs, the performance of WBMs, particularly when drilling through water sensitive rocks such as shales, is technically inferior to that of OBM. Shales exhibit great affinity for water, and adsorption of water by shales causes the shale to swell and produces chemical changes in the rock which produce stresses that weaken the formation, possibly leading to erosion of the borehole or loss of structure. This can lead to drilling problems such as wellbore erosion or stuck pipe. In addition, inferior wellbore quality may hinder logging and completion operations.
Much effort has been put into improving the performance of WBM relative to shales, namely improving the level of so called shale inhibition of WBM. Various chemical additives have been incorporated in WBM in attempts to improve shale inhibition. In particular water soluble glycols, polyhydric alcohols (i.e. chemicals containing more than one hydroxyl group) or polyglycols (i.e. chemicals made using alkylene oxides such as ethylene oxide or propylene oxide) are widely used for this purpose, typically being added to WBM in amounts in the range 3 to 10% by weight. These chemicals can be collectively referred to as Polyols. Polyols used in this way include, for example, glycerols, polyglycerols, glycols, polyalkylene glycols (PAG), e.g. polyethylene glycols (PEG), polypropylene glycols (PPG) and copolymers of ethylene and propylene glycols, alcohol ethoxylates (AET) and glycol ethers. A typical inhibitive AET is an n-butanol derivative of ethylene oxide. The PAGs can have a range of ethylene oxide:propylene oxide (EO:PO) ratios and can be random or block copolymers; a frequently used material of this type is understood to be a random copolymer with an EO:PO ratio of about 1:1.
Variants of polyalkylene glycols and alcohol alkoxylates are for example described in the International Patent Applications WO-96/24645 and WO-96/24646. Others are found in the European Patent Application EP-A-0495579, the U.S. Pat. Nos. 4,830,165 and 4,172,800.
A further source relating to the background of the invention is the Society of Petroleum Engineers Reports SPE 28960 (Mechanism of Shale Inhibition by Polyols in Water Based Drilling Fluids) proposing a credible mechanism that adequately describes how such polyols provide shale inhibition.
Copolymers of styrene and maleic anhydride and a number of derivatives are described for example in the U.S. Pat. No. 3,332,872, for use as viscosity control agents. Mydrophilic-hydrophobic graft copolymers with polystyrene side chains for drilling fluids appear in the U.S. Pat. No. 4,085,168. Other hydrophobically modified polymers based on polyacrylamide and hydroxyethylcellulose have been disclosed for example in the U.S. Pat. Nos. 5,597,783 and 5,637,556.
Shale swelling is considered as a problem not only in the oil field industry. It is encountered as clay swelling in the mining industry, where this phenomenon causes severe difficulties when dewatering the mineral tailings.
In view of the above, it is an object of the invention to provide a novel additive for inhibiting shale swelling. It is another, more specific object of the invention to provide an additive for a WBM.
The objects of the invention are achieved by shale-stabilizing additives as set forth in the appended independent claims. According to a first aspect of the invention, there is provided a shale-stabilizing additive comprising a polymer based on an olefinically unsaturated hydrocarbon with alkylene oxide based side chains.
Useful olefinically unsaturated hydrocarbons generally are aliphatic olefinically unsaturated hydrocarbons or vinylidene aromatic monomers.
Aliphatic olefinically unsaturated hydrocarbons include, for example, xcex1-olefin monomers containing from 2 to 28, preferably from 4 to 20, more preferably from 8 to 18 carbon atoms.
Preferred olefinically unsaturated hydrocarbons are vinylidene aromatic monomers. Suitable vinylidene aromatic monomers include, for example, those represented by the following formula. 
wherein R1 is hydrogen or an alkyl group containing 1 to 4 carbon atoms, preferably hydrogen or methyl; each R2 is independently hydrogen or an alkyl group containing 1 to 4 carbon atoms, preferably hydrogen or methyl; Ar is a phenyl group or a phenyl group substituted with 1 to 5 C1-C4-alkyl substituents; and n has a value from zero to 4, preferably from zero to 2, most preferably zero. Preferred monomers are styrene or styrene derivatives including xcex1-methyl styrene, the C1-C4-alkyl- or phenyl-ring substituted derivatives of styrene. such as ortho-, meta-, and para-methylatyrene, para-vinyl toluene or mixtures thereof. The most preferred vinylidene aromatic monomer is styrene.
The polymer is preferably a copolymer of an above-described olefinically unsaturated hydrocarbon and an ethylenically unsaturated carboxylic acid, carboxylic acid salt or carboxylic acid arhydride with alkylene oxide based side chains. Preferred olefinically unsaturated carboxylic acids contain 2 to 12, more preferably 2 to 6, carbon atoms in addition to the carboxyl group(s). They are preferably monocarboxylic acids or, more preferably, dicarboxylic acids. Preferred examples thereof are acrylic acid, methacrylic acid, fumaric acid, maleic acid, the salts thereof, or maleic anhydride. Most preferably, maleic acid, maleic anhydride and/or a maleic acid salt is incorporated in the polymer. The alkali metal salts, particularly the sodium salt, are the preferred carboxylic acid salts.
Alkylene oxide based side chains can be introduced into the polymer by esterification of carboxylic groups or anhydride groups with a polyoxyalkylated compound, such as a polyoxyalkylated alcohol. The esterification step can be carried out before or after the polymerization reaction of the dicarboxylic acid (salt) or the anhydride with the vinylidene aromatic monomer. The polymerization reaction and the esterification step are generally known in the art. Preferably 2 to 75 percent, more preferably 20 to 60 percent, most preferably about 50 percent of the total number of carboxyl carbon atoms in the polymer are esterified. In a preferred embodiment of the invention, the polymer is a copolymer of styrene or a styrene derivative and maleic anhydride with alkylene oxide based side chains.
The molecular weight of the additive is preferably from 5000 g/mol to 100,000 g/mol, more preferably from 10,000 g/mol to 30,000 g/mol. The preferred weight of the alkylene oxide based side chain in accordance with the invention is above 200 g/mol and below 1500 g/mol, more preferably 300-600 g/mol.
The number of alkoxylate groups, e.g. xe2x80x94CH2xe2x80x94CHRxe2x80x94Oxe2x80x94 with R being M, CH2 or CH2xe2x80x94CH3, per side chain is preferably up to 60 units.
A preferred molecule in accordance with the invention has the molecular structure: 
M in each occurrence independently is hydrogen or xe2x80x94[CH2xe2x80x94CHRxe2x80x94Oxe2x80x94]nxe2x80x94CH3 with R being CH3 or CH2xe2x80x94CH3 or, preferably, hydrogen, with the proviso that at least some of the radicals M have the meaning of xe2x80x94[CH2xe2x80x94CHRxe2x80x94Oxe2x80x94]nxe2x80x94CH3. X and Y each independently are from 1 to 100. The total of X and Y should generally be greater than 4 and the invention works best if the total of X and Y is greater than 50. The ratio of X:Y is preferably from 1:3 to 3:1, more preferably about 1:1. The value of n is from 3 to 70, preferably greater than 4 and less than 60. More preferably, n is 7-25.
The above-described polymer is preferably used as an additive in water-based drilling fluids (WBM) or completion fluids or in dewatering operations for mineral tailings.
In another aspect, the invention concerns a drilling fluid with 0.1 to 10 weight per cent (wt %) of the polymer as characterized before. More preferably, the polymer are used at 2 to 7 wt %.
The additives in accordance with the invention provide higher levels of shale swelling inhibition in the presence of weakly hydrated cations, such as potassium, cesium, and ammonium. Inhibition can be obtained solely from using the additives in freshwater or preferably in combination with salts containing so for example sodium, calcium, magnesium, or potassium ions.
It can also be expected that the novel additives can improve current methods of dewatering mineral tailings in the mining industry, as today this process is severely hampered by clay swelling.
These and other features of the invention, preferred embodiments and variants thereof, and further advantages of the invention will become appreciated and understood by those skilled in the art from the detailed description following below.